Chemistry Reference
In-Depth Information
3
Heterometallic Alkoxides
1
INTRODUCTION
Heterometal alkoxides represent a rapidly growing category of novel molecular species
in which alkoxo derivatives of two or more different metals are held together, generally
by alkoxo, chloro, or oxo bridging ligands.
As some rather diverse names are still in use for this interesting class of compounds, it
is appropriate to recapitulate some of them, particularly as the genesis of the current term
“heterometal alkoxides” itself has a historical perspective reflecting the gradual elucida-
tion of their chemical behaviour. In 1929, Meerwein and Bersin
1
reported the formation
of the so called “alkoxo salts”,
e.g.
K
C
fAl
OR
4
g
in the titrations of basic alkali
alkoxides with alkoxy derivatives of less electropositive metals in neutral solvents like
nitrobenzene. In 1958, Bartley and Wardlaw
2
showed that the alkali metal (M) zirconium
alkoxides actually had the formula NaZr
2
(OR)
9
instead of [NaHZr(OEt)
6
] reported by
Meerwein and Bersin. These NaZr
2
(OR)
9
derivatives were volatile covalent compounds.
A large number of similar derivatives were synthesized amongst others by Mehrotra and
co-workers, and a review article
3
entitled “Chemistry of Double Alkoxides of Various
Elements” appeared in 1971, followed by a general survey
4
in 1978 and another account
5
in 1983 dealing with “bimetallic alkoxides” of transition metals. The name “mixed
alkoxides” was also coined for these compounds by Chisholm and Rothwell
6
in 1988.
Compounds incorporating three different metals: [
Pr
i
O
2
Al
-OPr
i
2
Be
-OPr
i
2
Zr
OPr
i
3
]and[
Pr
i
O
2
Al
-OPr
i
2
Be
-OPr
i
2
Nb
OPr
i
4
] were first reported
7
in 1985
and these were called “trimetallic alkoxides”. This was soon followed in 1988 by the
work on tri- and tetrametallic alkoxides of copper
8
and other later 3d transition metals.
9
In 1987 Mehrotra
10
drew attention to potential applications of these easily purifiable
and soluble “bimetallic” and higher alkoxides as precursors for the preparation of glasses
and ceramic materials, which immediately brought forth the disclosure of an earlier
conjecture by Dislich.
11
All this finally led to the first publication entitled “Polymetallic
Alkoxides — Precursors for Ceramics”,
12
in which a variety of possible applications
of such species as precursors were spelled out by Mehrotra in 1988. In view of the
possibility of the term “polymetallic alkoxides” being confused with polymerized forms
of homo-metal systems, such as [M
OR
x
]
n
, the name was almost immediately changed
to “Heterometal alkoxides”, which has been gradually adopted since then in a number of
review articles.
13-29
In the present account, the term “heterometal alkoxides” has been
employed to represent these compounds in general, with bi-, tri-, and tetraheterometallic
alkoxides denoting molecular species containing two, three, or four different metals,
respectively.
